(1) Field of the Invention
This invention relates to an assembly and method for forming and subsequently heat treating articles of near net shape from powder metal.
Hot consolidation of metallic, intermetallic, and non-metallic powders and combinations thereof has become an industry standard. Hot consolidation can be accomplished by filling a container with a powder to be consolidated. The container is usually evacuated prior to filling and then hermetically sealed. Heat and pressure are applied to the filled and sealed container.This can be accomplished by pressing the container between the dies of a press while restraining the container to cause plastic flow of the container mass or it can be accomplished in an autoclave where gas pressure applies pressure over the surface of the container to cause plastic flow of the container material whereby the container shrinks or collapses. As the container shrinks or collapses the powder is densified. In other words, at elevated temperatures, the container functions as a pressure-transmitting medium to subject the powder to the pressure applied to the container. Simultaneously, the heat causes the powder to fuse by sintering. In short, the combination of heat and pressure causes consolidation of the powder into a substantially fully densified and fused mass in which the individual powder particles have lost their identity.
After consolidation, the container is removed from the densified powder compact and the compact is then further processed through one or more steps, such as forging, machining and/or heat treating, to form a finished part.
Due to difficulties encountered in post consolidation processing, efforts have been made to produce "near net shapes". As used herein, a near net shape is a densified powder metal compact having a size and shape which is relatively close to the desired size and shape of the final part. Producing a near net shape reduces the amount of post consolidation processing required to achieve the final part. For example, in many instances, subsequent hot forging may be eliminated and the amount of machining required may be significantly reduced.
(2) Description of the Prior Art
United States Pat. No. 4,142,888 granted Mar. 6, 1979 in the name of the inventor of the subject invention discloses a container for hot consolidation of powder wherein the container includes a mass of container material which is substantially fully dense and incompressible and is capable of plastic flow at pressing temperatures. A cavity of a predetermined shape is formed within the mass for receiving a quantity of powder and the mass includes walls around the cavity of sufficient thicknesses so that the exterior surface of the container does not closely follow the contour of the cavity so that upon application of heat and pressure to the container, the mass acts like a fluid to apply hydrostatic pressure to the powder contained in the cavity. As illustrated in that patent, the volume of the cavity is reduced as the walls of the cavity all move inwardly as the powder is compacted.
It is difficult to make the desired near net shapes when the compact or article has a complex shape. In order to obtain compacts or articles of complex shapes which are of near net shapes, it is sometimes necessary for a shaping portion of the container to extend into the cavity. During compaction this shaping portion moves with the interior walls of the cavity and may cause compaction of the powder on one side of the shaping portion before the compaction on the other thereby preventing the desired near net shape.